This invention relates generally to shoes. More specifically, it relates to a heel construction for a shoe characterized by a high degree of vertical compliance with substantially no lateral shear.
Recent work by applicants on the biomechanics of locomotion has led to the discovery that there is an optimal degree of "springiness" or vertical compliance which should be present at the interface between a person's foot and the surface on which he is walking or running. This discovery, which is discussed in more detail in applicants article, "Fast Running Tracks", appearing at pages 148-163 of the December, 1978 issue of Scientific American, contradicted the then conventional wisdom that a harder surface will produce a faster running time. Applicants construction for an indoor running track, now in use at Harvard University and pictured at page 163 of the Scientific American article, achieves this "tuned" response. In competitive running events, a principal advantage of a tuned surface is an increase in running speed. However, other important advantages are a reduction in the number of injuries associated with running and a general increase in the comfort of the runner. While ideally all surfaces, particularly all athletic playing surfaces, should provide this optimal degree of vertical compliance and the attendant advantages, this is, of course, not feasible. This is particularly true for amateur jogging where most running occurs on relatively hard surfaces such as concrete or asphalt sidewalks or roads.
In the manufacture of shoes, many arrangements have been used or suggested to cushion the shock of the foot striking the ground. A common expedient is simply to place a layer of resilient material in the shoe between the outer sole and the inner sole or the sock lining. There has been, however, no recognition that there is an optimal degree of vertical compliance for a shoe. Nor has there been any such arrangement which can provide a large degree of "cushioning" without compromising the performance of the shoe in other areas.
A fundamental design conflict is that a straightforward increase in the depth of the cushioning material results in an increase in its horizontal compliance or lateral shear. Horizontal compliance is undesirable because (1) it causes the foot to shift laterally with respect to the shoe at impact resulting in poor rearfoot stability and control, and (2) the energy transiently stored in the lateral deformation of this material is not returned to the runner. Thick cushioning layers also increase heel penetration, an undesirable vertical movement of the heel of the foot downwardly into the shoe on impact. These problems are accentuated in running shoes. The impact forces are much greater during running than walking and most amateur runners land on their heels rather than on the balls of their feet. During impact, the foot is at an angle with respect to the ground. The impact force therefore has a lateral or horizontal component, that is, a component directed generally along the sole of the shoe.
Another important design consideration is that the shoe construction should absorb as little as possible of the energy generated by the foot striking the ground. Stated in other words, the construction should transiently store and return energy to the runner efficiently. Prior art shoe constructions, in general, neither recognize this as a desirable goal, or achieve it. Conventional resilient cushioning materials absorb energy, typically dissipating it as heat. Thus, the runner loses a significant portion of his vertical kinetic energy every time his foot strikes the ground.
Some other practical design considerations include the weight of the heel, its height, its durability and its weight distribution. In competitive running, it has long been recognized that light weight shoes are preferable. Thus, there has been a steady reduction in the weight of running shoes over the years, due principally to the utilization of modern synthetic materials and advanced construction techniques. It is also recognized that there are practical constraints on the height of a shoe heel, particularly the heel of a competitive running shoe. Extremely tall heels, for example heels in excess of 1 and 1/4 inch are uncomfortable. Also, if the heel is formed of a resilient material, a tall heel exhibits a large lateral shear. Thus any practical heel design for a running shoe must be light weight, vertically compact, and rugged.
Most modern running shoes offer a relatively low degree of vertical compliance. The outer sole and heel are typically formed of a resilient material such as a high durometer polyurethane or a hard rubber. These materials are comparatively hard and stiff. Other layers forming the sole of the shoe typically include a layer of a more resilient material, but the composite structure remains, in general, comparatively hard and stiff. At the heel, the vertical compliance of almost all modern running shoes expressed as a spring constant (the inverse of compliance), is well in excess of 20,000 lbf/ft. At the front part of the shoe, for example at the ball of the foot, it is typically in excess of 35,000 lbf/ft.
Another known technique for providing cushioning is to form the outer sole of the shoe with a textured or ripple configuration. Such constructions, however, do not solve the aforementioned problems because (1) they do not provide an optimal degree of vertical compliance, (2) they suffer from lateral shear, and (3) they absorb the incident kinetic energy developed by the runner rather than efficiently returning it to him.
Another concept which appears in the prior art is to place a spring in the sole and/or heel of a shoe to provide cushioning. These spring designs, however, are deficient. None recognize that there is an optimal degree of vertical compliance for a given user and use. They merely recognize that some shock abosrbing cushioning is desirable. As to construction particulars, most of this prior art uses one or more coil or leaf springs located in the sole and/or heel of the shoe. One problem with these arrangements is that if the spring is large enough to provide a relatively large vertical compliance, then it is too heavy for use on a running shoe. Moreover, regardless of size, the springs depicted do not have enough vertical travel to store the large amount of energy developed during running. Further, while coil springs generally exhibit better energy storage characteristics then leaf springs, coil springs exhibit a large degree of lateral shear under a horizontal load. While some of the prior art patents disclose mechanical arrangements apparently intended to control the lateral shear of the coil spring or springs, they are generally heavy and impractical. A common such arrangement is to form the heel itself or spring support columns from two elements that are telescopically mounted for a vertical sliding movement.
Still another approach has been to utilize enclosed air as a cushioning medium. As with the spring patents, none of this "air cushion" prior art discloses any recognition that there is an optimal value for the vertical compliance of the shoe, particularly in its heel area. The air cushion is simply a shock absorber. While air has a great weight advantage over springs, air cushion designs suffer from a large degree of lateral compliance. Moreover, increasing the amount of the enclosed air or increasing the flexibility of the structure enclosing the air to increase the level of the vertical compliance accentuates the lateral shear problem. (This problem occurs even where the air is not entrapped, as, for example, where holes or channels are formed in the heel material to enhance its springiness and lower its weight.) Another problem is that the air cushions are inefficient in transiently storing energy. Energy from the runner is dissipated as heat rather than being returned to the runner.
It is therefore a principal object of this invention to provide a shoe construction, and in particular a heel construction for a shoe, that is biomechanically tuned to provide optimal performance characteristics for a variety of users and uses.
Another principal object of the invention is to provide a shoe construction that reduces the likelihood of injuries, particularly during running, or the aggravation of existing medical problems.
Another object of the invention is to provide a shoe that exhibits an extremely high degree of vertical compliance while at the same time exhibiting excellent rearfoot stability, rearfoot control, and a low level of heel penetration.
Still another object of the invention is to provide a shoe construction with replaceable heels to accomodate for wear and/or variations in the use of the shoe or the type of surface.
Yet another object of the invention is to provide a jogging shoe for use by amateur runners on sidewalks or hard surfaces as well as a training shoe for competitive runners that allows them to train harder with a reduced likelihood of injury.
Another object of the invention is to provide a competitive running shoe which can increase running speed on any surface.
Still a further object of this invention is to provide a shoe construction which is highly efficient in transiently storing and returning energy to the runner.
Another advantage of the invention is to provide a shoe construction with a comfortable heel height and which generally enhances the comfort of the person wearing the shoe.
Still another object of the invention is to provide a shoe construction having the foregoing advantages which can be manufactured from commonly available materials and uses conventional shoe uppers and soles.
Another object of the invention is to provide a heel construction for a shoe with the foregoing advantages that is comparatively light, durable, and has a competitive cost of manufacture.